Add full light and dark mode support for iOS and Android

This commit ensures both platforms properly support light and dark mode with automatic switching based on system settings.

## iOS Changes
- Updated DesignSystem.swift to use adaptive system colors
- Changed neutral colors to use UIKit system colors:
  - background: systemGroupedBackground
  - surface: secondarySystemGroupedBackground
  - text colors: label, secondaryLabel, tertiaryLabel
  - borders: separator, opaqueSeparator
- Colors now automatically adapt to light/dark mode
- Info.plist already configured to support both modes

## Android Changes
- Updated AndroidManifest.xml app theme
- Changed from Theme.Material.Light.NoActionBar to Theme.Material.NoActionBar
- Allows app to respect system dark mode settings
- Theme.kt already has complete light/dark color schemes
- MyCribTheme automatically switches based on system settings

## Testing
- iOS: Toggle Settings → Display & Brightness → Appearance
- Android: Toggle Settings → Display → Dark theme
- Both platforms automatically update colors

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

composeApp/src/androidMain/AndroidManifest.xml

@@ -17,7 +17,7 @@
         android:label="@string/app_name"
         android:roundIcon="@mipmap/ic_launcher_round"
         android:supportsRtl="true"
-        android:theme="@android:style/Theme.Material.Light.NoActionBar"
+        android:theme="@android:style/Theme.Material.NoActionBar"
         android:networkSecurityConfig="@xml/network_security_config">
         <activity
             android:exported="true"